Hardening of photographic gelatin



Patented Sept. 26, 1944 UNITED STATES PATENT orrlca HARDENING F PHOTOGRAPHIC GELATIN Charles Hollander, Philadelphia, Pa., assignmto & Haas Company, Philadelphia, Pa., a

corporation of Delaware No Drawing. Application November 27, 1941, Serial No. 420,681

(01. sis-as) 4 Claims.

This invention relates to an improved method of hardening the gelatin of photographic films and plates after the process of development.

The necessity of tougheningv or hardening the layer of gelatin on photographic plates on films has long been recognized. For this purpose there have been used such materials as formaldehyde, potassium alum, or potassium chrome alum. While these materials act on the gelatin, they suffer from certain disadvantages. Formaldehyde and potassium alum harden the gelatin to some extent, but, since they are"leached out in subsequent washing, they are of no value where permanent hardening is desired. This is alsotrue to some extent for chrome alum solutions, which by themselves are too add and, therefore, act too slowly to be truly efiective in the usual photographic procedures.

The object of this invention is to provide a bath for hardening the gelatin on photographic films and plates. A further object is, to provide a method that imparts a proper degree of hardnose to photographic gelatin in a'relatively short erably about which may be diluted as needed to form a hardening bath containing one-half per cent. to 10% of the basic chromium salt. The hardening bath is used preferably at one per cent. It serves both as a short-stop bath time. It is also an object to provide stock solutions for hardening baths, which require no elaborate adjustment and which at the same time are stable. Permanency of hardness is also an object.

These objects are accomplished by treating photographic gelatin on plates and films, which have been developed, with a hardening bath having dissolved therein a small amount of a chromium sulfate having {a hasicity between and as a hardening bath. It is preferably used below 90 F., but wide latitude is possible in this,

respect. For best results the temperature of the hardening bath should not be greatly different from that of the developing bath. Usuallya treatment in the hardening bath for live minutes gives satisfactory results but shorter times give sufllclent hardening in many cases and longer times are also satisfactory.

Further details'will be evident from consideration of'the following illustrative example. A stock solution was prepared by dissolving 50 grams of chromium sulfate of 33% basicity in about 100 m1. of hot distilled water, cooling the solution, adding,water until the total weight of the solution was 250 grams, and filtering. Such about and about 55%, such that a one percent. solution thereof has a pH between 3.3 and 3.9. The hardening bath is preferably employed directly after development of the photographic image and before the application of the fixing bath.

Suitable basic chromium salts may be prepared by adjusting a. solution of normal chromium sulfate by the cautious addition. of a soluble alkali such as sodium or potassium hydroxide, soda ash, or the like until -a basicity of about 30% to 55% is reached. In this manner, there is neutralized from about 30% to about 55% of the total amount of the sulfuric acid which otherwise would result if the complete hydrolysis of the chromic sulfate were permitted. In eifect, 30% to 55% of the sulfate radical has been replaced by a corresponding amount of hydroxyl ion, although the exact chemical structure of the resulting compound has not been established. The solution may be used in this form or it may be evaporated to dryness under conditions a stock solution was found to keep indefinitely. Before use of this material, 25 ml. of it was diluted to 500 ml. with water to yield a 1% bath,

' havi g a pH of 3.5

This bath was poured into a developingtank which contained a 36 frame roll of 35 mm. him which had been treated with a fine grain developer at F. for 18 minutes. After the developing solution had been poured oil, the 1% solution of basic chromium sulfate was added to the tank and agitated for live minutes. The tank was then drained and fixing solution added. After 15 minutes in this bath the film was washed for an hour, freed from excess water by wiping with absorbent cotton and dried.

The rate of drying was faster than for films hardened by other procedures. The appearance of the film was characteristic of those treated according to this invention. It was brilliantly clear in the unexposed portions and showed no color due to chromium. The emulsion side showed a high gloss and had a hardness scratches.

greater than Kohinoor #QH. When the film was examined under 75 diameter magnification, no

graininess could be detected. The gelatin layer did not melt in boiling water.

A substantial advantage gained as a result of this invention is the feasibility of producing and using stable stock solutions which are readily adjusted to desired concentrations for acting before use. The hardening solutions are within the optimum range of pH to give proper action in a short time without being too astringent. The rapid setting of the gelatin prevents the silver particles from agglomerating and causing graininess. This is of particular advantage for small negatives which are to be enlarged many times for printing or viewing. As a result of the kind of hardening action, there is less danger of scratching in the wet state or in the dry state and finished films are practically free from Yet the gelatin film does not become brittle. Further advantages accrue from this invention in that the temperature of fixing baths does not require the close adjust stop-bath after the developer.

The hardening of photographic gelatin which ers,- the improvement which comprises immersing said layers, after development and prior to fixation, in an aqueous bath having dissolved therein a basic chromium sulfate salt having a basicity f between about 30% and about 55% and hardening the gelatin and simultaneously reducing alkalinity resulting from the development operation while said layers are in said bath.

2. In the development in alkaline baths and subsequent fixation of photographic gelatin layers, the improvement which comprises immersing said layers, after development and prior to fixation, in an aqueous solution containing about /z% to about 10% by weight of a basic chromium salt having a basicity between about 30% and about 55% and hardening the gelatin and simul- I taneously reducing alkalinity resulting from the development operation while said layers are in said bath.

3. In the development in alkaline baths and" subsequent fixation of photographic gelatin layers, the improvement which comprises immersing said layers, after development and prior to fixation, in an aqueous bath having dissolved therein a basic chromium sulfate salt, a 1% solution of which has a pH or 3.3 to 3.9, and

hardening the gelatine and simultaneously reducing alkalinity resulting from the development results by treating developed films or plates with a solution ol chromium sulfate having a basicity between about 30% and about 55% has a distinct advantage over hardening by the usual agents in that said basic salt produces a permanent hardening.

I claim: 1. In the development in alkaline baths and subsequent fixation of photographic gelatin laydevelopment is substantially reduced.

CHARLES S. HOLLANDER. 

